1. Field of the Invention
This invention relates to promoting the growth, survival and productivity of bacteria for treating waste, and more particularly, to providing submerged fixed-film sites in combination with aerators for promoting bacterial growth in waste water, wherein the sites are organized in arrays for both efficient treatment operation and ease of maintenance.
2. Description of Prior Art
The use of inert support media as the locus of fixed film biomass growth dates to the 1930's. More recently, efforts have been made to use fixed bacterial film substrates in lagoons and other natural basins for holding liquid waste. In one example shown in U.S. Pat. No. 4,169,050 to S. A. Serfling issued Sep. 25, 1979, anchors such as elongated weighted members rest at spaced locations on the bottom of a pond or lagoon. Elongated buoyant substrates are provided with a pair of ends. A center of each elongated substrate is clipped to the anchor to allow the ends thereof to freely float toward the surface and form a buoyant, flexible permeable curtain. Between the spaced anchors, air diffusers are placed on the bottom of the lagoon to provide oxygen for metabolism of the bacteria.
Because the ends of the elongated substrates float freely in turbulence of the liquid waste induced by the bubbles, the substrates can become tangled during waste treatment. Even as important, upon draining the liquid waste from the lagoon during maintenance, the substrates can become tangled or become further tangled as the loose ends of the substrates settle at the bottom of the lagoon on the air diffusers. The substrates can thus interfere with efforts to clean and service the diffusers. When treatment plant personnel enter the lagoon to clean the air diffusers, they must first move the tangled substrates to expose the air diffusers. As a result, the maintenance process becomes disagreeable to the personnel, takes longer, and increases operational costs of treating the liquid waste.
In the late 1980's, the Environmental Protection Agency sponsored a study of a number of systems based on modifying activated sludge aeration tanks to provide various types of inert media to support fixed film biomass growth. It was reported that there appeared to be six high-biomass systems commercially available which could be incorporated into conventional aeration tanks. Two of such systems used highly reticulated sponges. One such system used a screen to keep the sponges in the aeration tank, and it was reported that among the problems that had to be solved were preventing blinding of the screen by using additional apparatus (an air-based system for returning the sponges to the influent volume of the tank).
In another of the systems reported on, the inert media was in the form of polyvinyl chlorodine (PVCE) strings having attached loops, with the strings wound around racks. The report noted that there was a persisting question as to the extent of stretching of the string, and additional structure such as a self-tensioning rack was said to address such stretching.
A final form of system studied in the EPA report used synthetic trickling filter media held on racks. It was noted plugging of this type of filter media has been of concern to certain individuals, and that an apparent way of negating this potential problem is to turn on an additional air blower periodically for short durations to slough off some of the fixed film biomass.
The study noted that additional throughput capacity realized by conversion to a high-biomass system could not be ascertained from the current data, but that it appeared that such conversion could at least double the biomass concentration and reduce by at least one-third the required return activated sludge pumping rate, as compared to a conventional activated sludge plant. The potential for increased throughput capacity was indicated, and there was a suggestion that high biomass systems may be more cost effective for plant upgrading than conventional approaches.
Another form of strings formed around racks is shown in U.S. Pat. No. 4,442,930 to Hatanaka, issued Dec. 27, 1983. Here, an elaborate frame system and rails provided at the top of a basin/pond are used to support upper and lower sections of movable rods. Long lengths of substrates are wound in a zig zag path around successive upper and lower ones of the rods. The rods are normally spaced across the length of a section of the frame, so that the zig zag substrates are in a large volume of the basin. The fixed zig zag path of the substrates avoids having freely floating loose ends of the substrates become entangled. The rods are spaced by bendable gap-retaining members which are held straight during operation of the system. To enable maintenance to be performed at the bottom of the basin, the gap-retaining members are bent to pull the rods to one side like a curtain and allow access to equipment below the substrates.
Although the frame, rails, and rods control the paths of the elongated substrates and avoid substrate tangling, the complexity of these heavy structural members significantly increases the installation cost. Further, the presence of the close-packed rods and substrates, and of the frame and rails, inherently makes it more difficult to access the entire bottom of the basin for maintenance.
In spite of these and other attempts to control the location of fixed-film submerged substrates, there is still a need for an inexpensive way to effectively position such substrates during liquid waste treatment while substantially reducing the interference of the substrates and their supports during maintenance.